This invention relates to an arm type valve gear used with an internal combustion engine.
Among arm type valve gears adapted to open and close an intake valve or an exhaust valve (hereinafter merely referred to as a valve) by rotating a cam, there are swing arm type valve gears, in which a camshaft is provided under a pivotally supported arm and the arm is pushed down by a cam mounted on the camshaft to open the valve, and rocker arm type valve gears, in which a camshaft is provided over an arm, and one end of the arm is pushed up to open a valve with the other end of the arm.
In these arm type valve gears, a valve clearance is automatically adjusted by mounting a lash adjuster.
As lash adjusters to be mounted in arm type valve gears, there are known hydraulic lash adjusters as described in JP patent publication 10-231711 and mechanical lash adjusters as described in JP utility model publication 5-21602.
In the hydraulic lash adjuster described in the former patent publication, since engine oil is used as hydraulic oil, there are the following problems.    {circle around (1)} Since engine oil is stirred by a crankshaft, bubbles tend to mix into the oil. If oil in which bubbles are mixed is supplied into a pressure chamber of the lash adjuster, the lash adjuster compresses bubbles when the valve is opened, so that the lift amount of the valve changes.    {circle around (2)} The amount of air dissolved in the engine oil increases with a change in temperature and pressure while the engine is running, so that air separates from the oil and accumulates in the pressure chamber due to cooling after the engine has stopped. This also causes the lift amount of the valve to change.    {circle around (3)} A reservoir chamber is needed to maintain the function of the lash adjuster until oil pressure is ensured at the restart of the engine, so that the lash adjuster tends to be large and heavy.    {circle around (4)} In order to introduce hydraulic oil into the lash adjuster, it is necessary to form a small-diameter oil supply passage in the arm, which is extremely difficult to form.
On the other hand, a mechanical lash adjuster does not have the problems encountered in hydraulic lash adjusters, and mechanical lash adjusters are extremely advantageous. But since female threads with which a male thread member is brought into engagement are formed on the inner periphery of a tubular lifter body having a bottom, it is necessary to provide a relief for a tool at the closed end of the lifter body in tapping the female threads. This increases the axial length of the lifter body and thus increases the size of the lash adjuster.
Further, since the valve stem is pushed down by the pivoting of the arm, it is necessary to form a spherical surface at the closed end of the lifter body and bring it into point contact with the top end face of the valve stem. Thus, the surface pressure between the lifter body and the valve stem is high, so that wear tends to develop at the contact portion.
An object of this invention is to provide an arm type valve gear in which a mechanical lash adjuster is mounted between an arm and a valve stem, and in which the lash adjuster is compact in its axial length and light in weight, and wherein wear at the contact portion with the valve stem is suppressed.